Fix raycast for plane geometry dev

The raycast logic with Face4 can break depending on the triangulation.  This is easily seen when basing some simple height mapped geometry on PlaneGeometry and modifying vertex positions since the planar assumption no longer holds.  By using Face3 instead of Face4, it will continue to work with raycasting in all cases.

See the example that I used to debug this here: http://www.realitymeltdown.com/WebGL/Debug-Raycast.html.